Methods for making environmental barrier coatings and ceramic components having CMAS mitigation capability

ABSTRACT

Methods of making components having calcium magnesium aluminosilicate (CMAS) mitigation capability involving providing a component; applying an environmental barrier coating to the component, the environmental barrier coating having a separate CMAS mitigation layer including a CMAS mitigation composition selected from rare earth elements, rare earth oxides, zirconia, hafnia partially or fully stabilized with alkaline earth or rare earth elements, zirconia partially or fully stabilized with alkaline earth or rare earth elements, magnesium oxide, cordierite, aluminum phosphate, magnesium silicate, and combinations thereof.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made, at least in part, with a grant from theGovernment of the United States (Contract No. N00019-04-C-0093, from theDepartment of the Navy). The Government may have certain rights to theinvention.

TECHNICAL FIELD

Embodiments described herein generally relate to methods for makingenvironmental barrier coatings and ceramic components having calciummagnesium aluminosilicate (CMAS) mitigation capability.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslybeing sought in order to improve their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. Significant advances inhigh temperature capabilities have been achieved through the formulationof iron, nickel, and cobalt-based superalloys. While superalloys havefound wide use for components used throughout gas turbine engines, andespecially in the higher temperature sections, alternativelighter-weight substrate materials have been proposed.

CMC and monolithic ceramic components can be coated with environmentalbarrier coatings (EBCs) to protect them from the harsh environment ofhigh temperature engine sections. EBCs can provide a dense, hermeticseal against the corrosive gases in the hot combustion environment. Indry, high temperature environments, silicon-based (nonoxide) CMCs andmonolithic ceramics undergo oxidation to form a protective silicon oxidescale. However, the silicon oxide reacts rapidly with high temperaturesteam, such as found in gas turbine engines, to form volatile siliconspecies. This oxidation/volatilization process can result in significantmaterial loss, or recession, over the lifetime of an engine component.This recession also occurs in CMC and monolithic ceramic componentscomprising aluminum oxide, as aluminum oxide reacts with hightemperature steam to form volatile aluminum species as well.

Currently, most EBCs used for CMC and monolithic ceramic componentsconsist of a three-layer coating system generally including a bond coatlayer, at least one transition layer applied to the bond coat layer, andan optional outer layer applied to the transition layer. Optionally, asilica layer may be present between the bond coat layer and the adjacenttransition layer. Together these layers can provide environmentalprotection for the CMC or monolithic ceramic component.

More specifically, the bond coat layer may comprise silicon and maygenerally have a thickness of from about 0.5 mils to about 6 mils. Forsilicon-based nonoxide CMCs and monolithic ceramics, the bond coat layerserves as an oxidation barrier to prevent oxidation of the substrate.The silica layer may be applied to the bond coat layer, or alternately,may be formed naturally or intentionally on the bond coat layer. Thetransition layer may typically comprise mullite, barium strontiumaluminosilicate (BSAS), and various combinations thereof, while theoptional outer layer may comprise BSAS. There may be from 1 to 3transition layers present, each layer having a thickness of from about0.1 mils to about 6 mils, and the optional outer layer may have athickness of from about 0.1 mils to about 40 mils.

Each of the transition and outer layers can have differing porosity. Ata porosity of about 10% or less, the layer is a hermetic seal to the hotgases in the combustion environment. From about 10% to about 40%porosity, the layer can display mechanical integrity, but hot gases canpenetrate through the coating layer damaging the underlying EBC. Whileit is necessary for at least one of the transition layer or outer layerto be hermetic, it can be beneficial to have some layers of higherporosity range to mitigate mechanical stress induced by any thermalexpansion mismatch between the coating materials and the substrate.

Unfortunately, deposits of CMAS have been observed to form on componentslocated within higher temperature sections of gas turbine engines,particularly in combustor and turbine sections. These CMAS deposits havebeen shown to have a detrimental effect on the life of thermal barriercoatings, and it is known that BSAS and CMAS chemically interact at hightemperatures, i.e. above the melting point of CMAS (approximately 1150°C. to 1650° C.). It is also known that the reaction byproducts formed bythe interaction of BSAS and CMAS can be detrimental to EBCs, as well assusceptible to volatilization in the presence of steam at hightemperatures. Such volatilization can result in the loss of coatingmaterial and protection for the underlying component. Thus, it isexpected that the presence of CMAS will interact with the EBC, therebyjeopardizing the performance of the component along with component life.

Accordingly, there remains a need for methods for making EBCs andceramic components having CMAS mitigation capability.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments herein generally relate to methods of making componentshaving CMAS mitigation capability comprising providing a component;applying an environmental barrier coating to the component, theenvironmental barrier coating comprising a separate CMAS mitigationlayer including a CMAS mitigation composition selected from the groupconsisting of rare earth elements, rare earth oxides, zirconia, hafniapartially or fully stabilized with alkaline earth or rare earthelements, zirconia partially or fully stabilized with alkaline earth orrare earth elements, magnesium oxide, cordierite, aluminum phosphate,magnesium silicate, and combinations thereof.

Embodiments herein also generally relate to method of making componentshaving CMAS mitigation capability comprising providing a component;applying an environmental barrier coating to the component, theenvironmental barrier coating comprising an integrated CMAS mitigationlayer including: BSAS; and a CMAS mitigation composition selected fromthe group consisting of rare earth elements, rare earth oxides, rareearth hafnates, rare earth zirconates, zirconia, hafnia partially orfully stabilized with alkaline earth or rare earth elements, zirconiapartially or fully stabilized with alkaline earth or rare earthelements, magnesium oxide, cordierite, magnesium aluminate spinel, rareearth monosilicates, rare earth disilicates, aluminum phosphate,magnesium silicate, alumina, and combinations thereof.

Embodiments herein also generally relate to methods of making componentshaving CMAS mitigation capability comprising providing a component;applying an environmental barrier coating to the component, the barriercoating comprising: a bond coat layer comprising silicon; an optionalsilica layer; at least one transition layer comprising a compositionselected from the group consisting of mullite, BSAS, and combinationsthereof; an optional outer layer comprising BSAS; and a CMAS mitigationcomposition wherein the CMAS mitigation composition is selected from thegroup consisting of rare earth elements, rare earth oxides, zirconia,hafnia partially or fully stabilized with alkaline earth or rare earthelements, zirconia partially or fully stabilized with alkaline earth orrare earth elements, magnesium oxide, cordierite, aluminum phosphate,magnesium silicate, and combinations thereof when the CMAS mitigationcomposition is included as a separate CMAS mitigation layer, and whereinthe CMAS mitigation composition is selected from the group consisting ofrare earth elements, rare earth oxides, rare earth hafnates, rare earthzirconates, zirconia, hafnia partially or fully stabilized with alkalineearth or rare earth elements, zirconia partially or fully stabilizedwith alkaline earth or rare earth elements, magnesium oxide, cordierite,magnesium aluminate spinel, rare earth monosilicates, rare earthdisilicates, aluminum phosphate, magnesium silicate, alumina, andcombinations thereof when the CMAS mitigation composition is included asan integrated CMAS mitigation layer further comprising BSAS.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating in accordance with the description herein;

FIG. 2 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating having a CMAS mitigation layer inaccordance with the description herein; and

FIG. 3 is a schematic cross sectional view of one embodiment of anenvironmental barrier coating having an integrated CMAS mitigation layerin accordance with the description herein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to environmental barriercoatings and ceramic components having CMAS mitigation capability.

The CMAS mitigation compositions described herein may be suitable foruse in conjunction with EBCs for substrates comprising CMCs, andmonolithic ceramics. As used herein, “CMCs” refers tosilicon-containing, or oxide-oxide, matrix and reinforcing materials.Some examples of CMCs acceptable for use herein can include, but shouldnot be limited to, materials having a matrix and reinforcing fiberscomprising non-oxide silicon-based materials such as silicon carbide,silicon nitride, silicon oxycarbides, silicon oxynitrides, and mixturesthereof. Examples include, but are not limited to, CMCs with siliconcarbide matrix and silicon carbide fiber; silicon nitride matrix andsilicon carbide fiber; and silicon carbide/silicon nitride matrixmixture and silicon carbide fiber. Furthermore, CMCs can have a matrixand reinforcing fibers comprised of oxide ceramics. These oxide-oxidecomposites are described below.

Specifically, the oxide-oxide CMCs may be comprised of a matrix andreinforcing fibers comprising oxide-based materials such as aluminumoxide (Al₂O₃), silicon dioxide (SiO₂), aluminosilicates, and mixturesthereof. Aluminosilicates can include crystalline materials such asmullite (3Al₂O₃ 2SiO₂), as well as glassy aluminosilicates.

As used herein, “monolithic ceramics” refers to materials comprisingonly silicon carbide, only silicon nitride, only alumina, only silica,or only mullite. Herein, CMCs and monolithic ceramics are collectivelyreferred to as “ceramics.”

As used herein, the term “barrier coating(s)” refers to environmentalbarrier coatings (EBCs). The barrier coatings herein may be suitable foruse on ceramic substrate components 10 found in high temperatureenvironments, such as those present in gas turbine engines, for example,combustor components, turbine blades, shrouds, nozzles, heat shields,and vanes. “Substrate component” or simply “component” refers to acomponent made from “ceramics,” as defined herein.

More specifically, EBC 12 may generally comprise any existingenvironmental barrier coating system that generally comprises a siliconbond coat layer 14, an optional silica layer 15 adjacent to bond coatlayer 14, at least one transition layer 16 adjacent to bond coat layer14 (or silica layer 15 if present), an optional outer layer 18 adjacentto transition layer 16, and an optional abradable layer 22 adjacent totransition layer 16 (or outer layer 18 if present), as shown generallyin FIG. 1. As defined previously herein, “transition layer” 16 refers toany of mullite, BSAS, and various combinations thereof, while “outerlayer” 18 refers to BSAS.

Unlike existing EBCs, and in addition to the layers describedpreviously, the present embodiments may also include CMAS mitigationcompositions to help prevent the EBC from degradation due to reactionwith CMAS in high temperature engine environments. Such CMAS mitigationcompositions may be present as a separate CMAS mitigation layer on topof the existing EBC systems, as a grain boundary phase in the BSAS outerlayer, or as discrete dispersed refractory particles in the BSAS outerlayer, as defined herein below.

As shown in FIG. 2, when CMAS mitigation is included in the EBC as aseparate mitigation layer on top of existing systems, “separate CMASmitigation layer” 20 may comprise at least one CMAS mitigationcomposition. As used herein throughout, “CMAS mitigation composition(s)”refers to compositions selected from rare earth elements (Ln), rareearth oxides, zirconia, hafnia partially or fully stabilized withalkaline earth or rare earth elements, zirconia partially or fullystabilized with alkaline earth or rare earth elements, magnesium oxide,cordierite, aluminum phosphate, magnesium silicate, and combinationsthereof. When included as an integrated CMAS mitigation layer, asexplained below, the “CMAS mitigation compositions” may also comprisealumina, rare earth hafnates, rare earth zirconates, rare earthmonosilicates, and rare earth disilicates.

As used herein throughout, “Ln” refers to the rare earth elements ofscandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium(Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and mixtures thereof.Furthermore, as used herein, “rare earth oxides” can refer to Sc₂O₃,Y₂O₃, CeO₂, La₂O₃, Pr₂O₃, Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃,Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃ or mixtures thereof.

By way of example and not limitation, when including a separate CMASmitigation layer 20, the EBC may comprise one of the followingarchitectures: a silicon bond coat layer 14, an optional silica layer15, a mullite transition layer 16, a BSAS outer layer 18, a separateCMAS mitigation layer 20, and optionally, an abradable layer 22; asilicon bond coat layer 14, an optional silica layer 15, a mullite-BSAStransition layer 16, a BSAS outer layer 18, a separate CMAS mitigationlayer 20, and optionally, an abradable layer 22.

In the previous examples, optional abradable layer 22 may comprise thesame material present in separate CMAS mitigation layer 20, a rare earthdisilicate (Ln₂Si₂O₇), or BSAS. The abradable may be a highly porouslayer comprising up to about 50% porosity, or it may consist ofpatterned ridges that are dense (less than about 10% porosity) or porous(up to about 50% porosity). Abradable layer 22 can abrade upon impactfrom an adjacent, rotating engine component. The energy absorbed intothe abradable coating can help prevent damage from incurring to theadjacent, rotating engine component. For example, in one embodiment, theEBC plus abradable layer could be present on a CMC shroud. Adjacentrotating blades having a tight clearance with the shroud could result inan impact event. The presence of abradable layer 22 can help preventdamage to the rotating blades.

As shown in FIG. 3, and as previously described, CMAS mitigation mayalternately be included as an integrated CMAS mitigation layer 120. Inthis instance, “integrated CMAS mitigation layer” 120 refers to a layercomprising CMAS mitigation compositions in combination with BSAS. Moreparticularly, the CMAS mitigation composition can be included as eitherdiscrete dispersed refractory particles in BSAS, or as a grain boundaryphase in BSAS.

When included as an integrated CMAS mitigation layer, the “integratedCMAS mitigation layer” may comprise BSAS with the addition of any of theCMAS mitigation compositions previously defined herein, as well asalumina, rare earth hafnates (Ln₂Hf₂O₇), rare earth zirconates(Ln₂Zr₂O₇), rare earth monosilicates, and rare earth disilicates.

By way of example and not limitation, EBCs having an integrated CMASmitigation layer 120 as either as a grain boundary phase or as discretedispersed refractory particles, may comprise one of the followingarchitectures: a silicon bond coat layer 14, an optional silica layer15, a mullite-BSAS transition layer 16, and an integrated CMASmitigation layer 120; a silicon bond coat layer 14, an optional silicalayer 15, a mullite transition layer 16, and an integrated CMASmitigation layer 120.

Regardless of the particular architecture of the EBC with CMASmitigation capability, the substrate component can be coated usingconventional methods known to those skilled in the art to produce alldesired layers and selectively place the CMAS mitigation compositions aseither a separate layer, as grain boundary phase in BSAS, or asdiscrete, dispersed refractory particles in BSAS. Such conventionalmethods can generally include, but should not be limited to, plasmaspraying, high velocity plasma spraying, low pressure plasma spraying,solution plasma spraying, suspension plasma spraying, chemical vapordeposition (CVD), electron beam physical vapor deposition (EBPVD),sol-gel, sputtering, slurry processes such as dipping, spraying,tape-casting, rolling, and painting, and combinations of these methods.Once coated, the substrate component may be dried and sintered usingeither conventional methods, or unconventional methods such as microwavesintering, laser sintering or infrared sintering. Unless an abradablelayer is present, the CMAS mitigation layer, whether separate orintegrated, can be the outermost layer of the EBC.

More specifically, the CMAS mitigation grain boundary phase can beproduced in a variety of ways, including particle coating and slurrymethods. In one example, the CMAS grain boundary phase can be achievedby coating particles of BSAS with the desired CMAS mitigationcomposition(s) before the BSAS is deposited on the ceramic substrateusing a conventional method known to those skilled in the art. Coatingthe BSAS particles can be accomplished by chemical vapor deposition onparticles in a fluidized bed reactor or by a solution (sol-gel) typeprocess where precursors of the CMAS mitigation composition aredeposited onto the BSAS particles from a liquid phase, followed by heattreatment of the BSAS particles to form the desired CMAS mitigationcomposition on the surface of the BSAS particles. Once the BSASparticles with the CMAS mitigation composition are obtained, thesubstrate component can be coated, dried, and sintered using any of thepreviously described methods known to those skilled in the art.Ultimately, the surface layer of the CMAS mitigation composition on theBSAS particles becomes the grain boundary phase in the coating. In theseinstances, to form the grain boundary phase, the refractory particlescan have an average size of less than about 100 nm. If the refractoryparticles are larger than about 100 nm, the particles will be too largeto properly coat the EBC particles as needed to form the grain boundaryphase, and will instead be present as dispersed refractory particles asexplained below.

In another example using slurry methods, BSAS particles (without a CMASmitigation composition coating) can be used. In such instances, thegrain boundary phases can be achieved by using sol-gel precursors in theslurry, by infiltration of sol-gel precursors into the dry coatings, orby infiltration of sol-gel precursors into the sintered coatingsfollowed by an additional sintering step.

Dispersion of the refractory particles into the BSAS layer can occur byvarious means depending on the process chosen to deposit the barriercoating. For a plasma spray process, BSAS particles can be mixed withthe CMAS mitigation refractory particles before coating deposition.Mixing may consist of combining BSAS and the refractory particleswithout a liquid, or by mixing a slurry of the BSAS and refractoryparticles. The dry particles or slurries can then be mechanicallyagitated using a roller mill, planetary mill, blender, paddle mixer,ultrasonic horn, or any other method known to those skilled in the art.For the slurry process, the refractory particles dispersed in the slurrywill become dispersed particles in the coating after drying andsintering of a slurry-deposited layer.

In order to maintain discrete, refractory particles in themicrostructure, the average particle size of the CMAS mitigationrefractory particles in the slurry can be greater than about 20 nm, andin one embodiment from about 200 nm to about 10 micrometers in size. Therefractory particles can comprise from about 1% to about 60% by volumeof the layer, with the remainder being BSAS or a combination of BSAS andporosity.

Regardless of whether the CMAS mitigation composition is present as aseparate mitigation layer on top of the existing EBC systems, or as anintegrated mitigation layer (e.g. discrete dispersed refractoryparticles in the BSAS, or a grain boundary phase in the BSAS), thebenefits are several. Namely, the CMAS mitigation compositions can helpprevent the EBC from degradation due to reaction with CMAS in hightemperature engine environments. More particularly, the CMAS mitigationcompositions can help prevent or slow the reaction of CMAS with thebarrier coating that can form secondary phases that rapidly volatilizein steam. Additionally, CMAS mitigation compositions can help prevent orslow the penetration of CMAS through the barrier coating along the grainboundaries into a nonoxide, silicon-based substrate. Reaction of CMASwith substrates such as silicon nitrate and silicon carbide evolvenitrogen-containing and carbonaceous gases, respectively. Pressure fromthis gas evolution can result in blister formation within the EBCcoating. These blisters can easily rupture and destroy the hermetic sealagainst water vapor provided by the EBC in the first instance.

The presence of CMAS mitigation compositions can help prevent or slowthe attack of molten silicates on the EBC, thereby allowing the EBC toperform its function of sealing the CMC from corrosive attack in hightemperature steam. Moreover, the CMAS mitigation compositions can helpprevent recession of the CMC, and also any layers of the EBC that may besusceptible to steam recession if CMAS reacts with it, to formsteam-volatile secondary phases. Dimensional changes of ceramiccomponents due to steam recession can limit the life and/orfunctionality of the component in turbine engine applications.

With respect to integrated CMAS mitigation layers in particular, havingCMAS mitigation compositions situated as grain boundaries or dispersedparticles can offer some unique benefits. For instance, the use ofintegrated CMAS mitigation layers can allow the EBC to be made withfewer overall layers and processing steps yet still provide the sameprotection to the underlying substrate component. Integrated layers useless CMAS mitigation composition to achieve protection from CMAScorrosion, which can result in cost savings. Moreover, integrated layersallow for the incorporation of desired properties of the outer layercompositions (i.e. the tendency of BSAS toward forming an excellent sealat high temperatures) and the CMAS corrosion resistance in a singularlayer.

Thus, CMAS mitigation, whether separate or integrated, is important toallow the barrier coating to perform its functions; thereby allowing theCMC component to function properly and for its intended time span.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A method of making a component having CMASmitigation capability comprising: providing a component, wherein thecomponent comprises a ceramic matrix composite or a monolithic ceramic;applying an environmental barrier coating to the component, theenvironmental barrier coating comprising: a bond coat layer comprisingsilicon overlying the component; an optional silica layer overlying thebond coat layer; at least one transition layer overlying the bond coatlayer or the optional silica layer comprising a composition selectedfrom the group consisting of mullite, BSAS, and combinations thereof; anoptional outer layer comprising BSAS overlying the at least onetransitional layer; an integrated CMAS mitigation layer overlying the atleast one transition layer or the optional outer layer, the integratedCMAS mitigation layer consisting of: BSAS; and a CMAS mitigationcomposition selected from the group consisting of magnesium oxide,cordierite, and combinations thereof; wherein at least a portion of theenvironmental barrier coating is susceptible to infiltration by CMAS,the BSAS in at least one of the transition layer, outer layer andintegrated CMAS mitigation layer is susceptible to reacting with CMASthat has infiltrated thereto, and the integrated CMAS mitigation layercomprises the CMAS mitigation compositions as a grain boundary phase inthe BSAS or as dispersed refractory particles in the BSAS.
 2. The methodof claim 1 wherein the component is a turbine engine component selectedfrom the group consisting of combustor components, turbine blades,shrouds, nozzles, heat shields, and vanes.
 3. The method of claim 2further comprising applying an abradable layer to the integrated CMASmitigation layer.
 4. The method of claim 3 comprising applying theenvironmental barrier coating using a method selected from the groupconsisting of plasma spraying, high velocity plasma spraying, lowpressure plasma spraying, solution plasma spraying, suspension plasmaspraying, chemical vapor deposition, electron beam physical vapordeposition, sol-gel, sputtering, slurry dipping, slurry spraying, slurrypainting, slurry rolling, tape-casting, and combinations thereof.